Oscillators are commonly used in applications such as refresh clock sources for dynamic random access memories (DRAM), watch-dog timer clock sources, time monitors, and the like. It is desirable that the oscillators provide stable frequencies under a wide range of working voltages, or in other words, independent from the power supply voltages.
There are various methods for providing the VDD-independent oscillators. For example, FIG. 1 illustrates a block diagram of a conventional VDD-independent oscillator. A power supply voltage VDDext (external VDD) is provided to a voltage down converter (VDC), which generates an internal voltage VDDint. The internal voltage VDDint is then provided to a ring oscillator, which then generates an oscillating signal OSC. Although the internal voltage VDDint is relative stable, it may still have about 10 mV to about 50 mV variation depending on the design of the VDC. Such variation also affects the accuracy of the frequency of the ring oscillator. In addition, even if internal voltage VDDint is a constant voltage, the ring oscillator still suffers from process variations, such problem cannot be solved by the constant VDDint. A further problem is that due to the use of the VDC, the chip area usage and the current consumption of the respective oscillator increase.
U.S. Pat. No. 5,352,934 provides a VDD-independent oscillator circuit, as is shown in FIG. 2, including a bandgap reference generator and a comparator. However, the circuit has a complicated design from using many devices. The chip-area usage and current consumption are thus high. In addition, since the bandgap reference generator uses bipolar junction transistors, its performance will be adversely affected when the oscillator is operated under sub-1V Vdd voltages. What is needed, therefore, is an oscillator overcoming the above-described shortcomings in the prior art.